Plotter right 99% of the time but you can have problems closer to home than the Gambia...
Entering the Morlaix estuary I would have hit the Chateau de Taureau rocks if I had believed my plotter (Raymarine/Navionics) and further up the river (and the Trieux river) I found GPS/Plotter unreliable, showing you up the hill.
Moral is to keep your wits about you and avoid tight entrances in fog unless you can rely on other methods e.g. radar, depth-sounder etc.
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